The present technology relates to a process for preparing stable high solids content aqueous emulsions.
Aqueous emulsions containing various homopolymers and copolymers, e.g., homopolymers and copolymers of vinyl esters, homopolymers and copolymers of lower alkyl acrylates and lower alkyl methacrylates, such as ethyl acrylate and methyl methacrylate, and the like, have been known for many years. The procedures used to prepare such polymer emulsions generally involve adding, with rapid stirring, one or more ethylenically unsaturated monomers to water which contains or to which is added, either simultaneously or subsequently, surfactants or emulsifying agents, a polymerization catalyst or initiator and, in many cases, a protective colloid-forming substance. This mixture is then heated to polymerization temperature with continued stirring, and held at that temperature until substantially complete polymerization of the monomer or monomers has occurred. The resulting polymer emulsion, upon cooling and filtering, can be used in many domestic and industrial applications, such as in paints or other coating compositions (e.g., paper coatings and textile-treating compositions), in adhesives or binders, in caulking compositions, and the like, depending on the particular polymers involved and the properties of the emulsions containing them.
The stability of an aqueous emulsion (i.e., an aqueous colloidal dispersion of a polymer) against coagulation or precipitation of its dispersed polymer component depends to some degree upon the kind and amount of surfactants, and protective colloids and the like which are contained in the aqueous emulsion medium. In many cases the aqueous emulsion is quite unstable, particularly when subjected to mechanical agitation, to heat or cold, or upon addition to the dispersion of polyvalent ions such as ionized calcium salts. In some instances, the stability of the latex can be improved by adding further amounts of surfactants or other stabilizers to the colloidal dispersion after completion of the polymerization step. However, latexes stabilized with external surfactants are stable only to a limited degree and are vulnerable to external destabilization.
Also, for many purposes water-soluble surfactants and stabilizers when added to increase emulsion stability are ultimately objectionable. Thus, in coatings derived from aqueous emulsions by evaporation of water from a deposite of such emulsions on a substrate, the non-volatile surface active agents tend to have a deleterious effect on the quality of the coatings.
Accordingly, it is an object of this invention to provide aqueous emulsion compositions which exhibit excellent stability against coagulation or precipitation of its dispersed polymer phase.
It is another object of this invention to provide a process for preparing stable high solids content aqueous emulsions which are adapted for application as paper coating binders.
It is a further object of this invention to provide latex coating compositions which exhibit improved wet abrasion resistance properties when coated on paper substrates.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.